1. Field of the Invention
This invention relates to the use of magnetic tunnel junctions (MTJ) as storage elements (cells) in non-volatile magnetic memory cell arrays (MRAM). In particular it relates to MRAM arrays of shielded MTJ cells in which the cells have their uncompensated edge poles eliminated by magnetostatic coupling to magnetic shields and, in addition, are shielded from each other and from extraneous external magnetic fields by various forms and configurations of said shields.
2. Description of the Related Art
The magnetic tunnel junction (MTJ) basically comprises two electrodes, which are layers of ferromagnetic material, separated by a tunnel barrier layer, which is a thin layer of insulating material. The tunnel barrier layer must be sufficiently thin so that there is a probability for charge carriers (typically electrons) to cross the layer by means of quantum mechanical tunneling. The tunneling probability is spin dependent, depending on the orientation of the spin of the tunneling electrons relative to the magnetization direction of the ferromagnetic layers. Thus, if these magnetization directions are varied, the tunneling current will also vary as a function of the relative directions for a given applied voltage. As a result of the behavior of an MTJ, sensing the change of tunneling current for a fixed potential can enable a determination of the relative magnetization directions of the two ferromagnetic layers that comprise it. Equivalently, the resistance of the MTJ can be measured, since different relative magnetization directions will produce different resistances.
The use of an MTJ as an information storage device requires that the magnetization of at least one of its ferromagnetic layers can be varied relative to the other and also that changes in the relative directions can be sensed by means of variations in the tunneling current or, equivalently, the junction resistance. In its simplest form as a two state memory storage device, the MTJ need only be capable of having its magnetizations put into parallel or antiparallel configurations (writing) and that these two configurations can be sensed by tunneling current variations or resistance variations (reading). In practice, the free ferromagnetic layer can be modeled as having a magnetization which is free to rotate but which energetically prefers to align in either direction along its easy axis (the direction of magnetic crystalline anisotropy). The magnetization of the fixed layer may be thought of as being permanently aligned in its easy axis direction. When the free layer is anti-aligned with the fixed layer, the junction will have its maximum resistance, when the free layer is aligned with the fixed layer, the minimum resistance is present. In typical MRAM circuitry, the MTJ devices are located at the intersection of current carrying lines called word lines and bit lines (or word lines and sense lines). When both lines are activated, the device is written upon, ie, its magnetization direction is changed. When only one line is activated, the resistance of the device can be sensed, so the device is effectively read. In this regard, Bronner et al. (U.S. Pat. No. 6,242,770 B1) teaches a method for forming thin film conductors as word and bit lines so that the MTJ device is in close proximity to a lower line and a diode is located below that line.
In order for the MTJ MRAM device to be competitive with other forms of DRAM, it is necessary that the MTJ be made very small, typically of sub-micron dimension. Parkin et al. (U. S. Pat. No. 6,166,948) notes that sub-micron dimensions are needed to be competitive with DRAM memories in the range of 10-100 Mbit capacities. Parkin also notes that such small sizes are associated with significant problems, particularly super-paramagnetism, which is the spontaneous thermal fluctuation of magnetization produced by in samples of ferromagnetic material too small to have sufficient magnetic anisotropy (a measure of the ability of a sample to maintain a given magnetization direction). Another size-related problem results from non-uniform and uncontrollable edge-fields produced by shape-anisotropy (a property of non-circular samples). These edge fields result in a large degree from randomly oriented magnetization vectors that form at the edges of the MTJ cells. These orientations have a tendency to curl back towards the magnetization vector of the body of the cell in an effort to minimize the magnetic energy of the cell. Such edge effects are also associated with uncompensated magnetic poles that form at the cell edges. As the cell size decreases, these edge fields become relatively more important than the magnetization of the body of the cell and have an adverse effect on the storage and reading of data. Although such shape-anisotropies, when of sufficient magnitude, reduce the disadvantageous effects of super-paramagnetism, they have the negative effect of requiring high currents to change the magnetization direction of the MTJ for the purpose of storing data. To counteract these edge effects, Shi et al. (U.S. Pat. No. 5,757,695) teaches the formation of an ellipsoidal MTJ cell wherein the magnetization vectors are aligned along the length (major axis) of the cell and which do not present variously oriented edge domains, high fields and poles at the ends of the element.
MTJ devices have been fabricated in several configurations, one type comprising a free ferromagnetic layer separated from a fixed (or pinned) layer. In such a configuration, the MTJ has data stored in it by causing the magnetization of its free layer to be either parallel or antiparallel to that of the pinned layer. The pinned layer may itself be a composite layer formed of two ferromagnetic layers held in an antiparallel magnetization configuration by some form of magnetic coupling so that it presents a zero or negligible net magnetic moment to the MTJ. Such an arrangement is advantageous in reducing edge effects due to anisotropies. Parkin, cited above, teaches an improved MTJ cell utilizing a free layer that comprises two ferromagnetic layers that are coupled by their dipolar fields in an antiparallel magnetization configuration to produce a small, but non-zero, magnetic moment. When written on by an external applied magnetic field, the two magnetic moments switch directions simultaneously so that the net magnetic moment of the free layer switches direction relative to the pinned layer. In addition, Gallagher et al. (U.S. Pat. No. 5,650,958) teaches the formation of an MTJ device suitable for use in an MRAM array wherein the device comprises a free ferromagnetic layer and a pinned ferromagnetic layer which is pinned by interfacial exchange with an antiferromagnetic layer. Gallagher et al. (U.S. Pat. No. 5,841,692) also teaches the formation of an MTJ device having free and fixed layers wherein the fixed layer is formed as a sandwich of antiferromagnetically coupled ferromagnetic layers. Further, Shi et al. (U.S. Pat. No. 5,959,880) teach the formation of a low aspect ratio MTJ device in which two layers of magnetoresistive material are separated by electrically insulating material.
It is undesirable for MTJ devices to have excessive magnetic coupling between adjacent magnetic layers of neighboring devices or even within the same device as this coupling must be overcome when writing on the device. As noted above, edge anisotropies are one source of undesirable coupling. Koch et al. (U.S. Pat. No. 6,005,800) deal with the problem that results when writing to one specific cell also affects the magnetization directions of adjacent cells that are not being addressed. Koch teaches the formation of cells with two shapes, which are mirror images of each other. The cells are arranged in a checkerboard pattern, so that a cell of one shape is surrounded by cells of the other shape. Since neighboring cells thereby have their preferred magnetization vectors oriented differently, there is a reduced probability that writing to one cell type will affect the magnetization of the other type.
As has been discussed, many of the problems associated with the construction of MRAM arrays are related to the shapes of the cells. Cell shapes of present designs are typically single element rectangle, elliptical or lozenge. Chen et al. (U.S. Pat. No. 5,917,749) provides a rectangular multi-layered MTJ cell comprising two rectangular magnetic layers magnetized in parallel directions along an easy axis corresponding to a direction of magnetic anisotropy and separated by a non-magnetic layer.
Any irregularities of these shapes, defects at their edges produced during their formation, or uncompensated poles of variable strength, will result in coercivity fluctuations distributed throughout the array. It is the object of the present invention to control the problem of undesirable edge effects and non-uniform array coercivity more effectively than is done in the prior art by providing magnetic shields between arrangements of cells within an MRAM array. These shields serve several purposes, including providing pole compensation for edge poles of cell elements, shielding cells from the effects of external magnetic fields and shielding the magnetizations of individual cells from the effects of nearby cells. The use of shields to partially surround MTJ cells is not unknown in the prior art. Gill et al. (U.S. Pat. No. 6,219,212 B1) provide an MTJ device for use as an MRAM cell or as a magnetic field sensor in a magnetic disk drive, in which magnetic material layers disposed above and below the MTJ device. The shields also act as current leads for the MTJ device. It is evident from the topology of the shielding layers that they are not intended to shield one such MTJ device from coplanar adjacent MTJ devices. Furthermore, it is also evident from the shield topology that they are not intended to cancel uncompensated poles formed at the edges of the magnetic free layer of the MTJ device.